The present invention relates to a time synchronization technology using a satellite. More specifically, this invention relates to a time synchronization system for synchronizing the time in the satellite system with the time in the ground system, a satellite system applied in the time synchronization system, a ground system applied in the time synchronization system, a time synchronization method, and a computer-readable recording medium with a program for making a computer execute the method.
When astronomical events are observed using a remote system such as a satellite, it is required to synchronize the time in the satellite system with that of the ground time in order to know the time of generation of observed events. With recent advance in the field of computers, high speed data processing has become possible in the satellite system, and also an advanced protocol such as packet telemetry based on recommendation by CCSDS (Consultative Committee for Space Data Systems) for communications between a satellite system and a ground station are used.
Because of the necessity of data processing in a satellite or employment of packet telemetry, ambiguous delay in time interval is generated from the time when data is acquired until the time when the data is transmitted to the ground. Therefore, it is difficult to estimate the time of generation of an event from a time when the data is received at the ground station. On the other hand, it is required to know the accurate time of generation of data while realizing a high speed data processing in the satellite system or an advanced protocol such as packet telemetry. Especially in astronomical observation, in order to verify the result of observation to that acquired by other satellites or that obtained on the ground, precision in time measurement of the order of microseconds is required.
In the conventional technology, observed data is sampled according to the timing generated by an apparatus for controlling the timing of operations of the entire satellite system. The data is inserted at a fixed location in the transfer frame and the time at which the data was generated is determined from the time when the transfer frame is received. On the other hand, a system in which request for the current time is made when required to a device which controls the system time, or a system in which standard time is determined by using data input time into a device generating a transfer frame is employed in a satellite system employing the packet telemetry therein.
In recent years, time is determined by using the GPS (Global Positioning System). FIG. 15 shows the commonly used GPS system. The system shown in FIG. 15 is a satellite system in which an orbiting satellite 105 acquires the signals, namely the observation data, from four GPS satellites 101 to 104. In this satellite system, the satellite 105 acquires apparent distances between itself and the GPS satellites 101 to 104 based on the acquired observation data, and obtains four unknown parameters i.e. its own position (x, y, z) and the difference between its own time and the time in the GPS satellites. With this method, an accurate time can be acquired in the satellite 105.
In the satellite system described above, in association with the advance in the computer technology, sophisticated processing such as data compression or data extraction has become possible, so that data length of the observed data or the like changes, and sometimes waste of resources occurs in data transfer when a fixed data format like that in the conventional technology is used. Efforts have been made in order to improve the efficiency in data transmission by employing packet telemetry such as CCSDS.
However, in the satellite system described above, because the complicated data processing such as the packet telemetry like CCSDS is employed, a delay is generated until the acquired data is packetized, or ambiguous delay is generated until a packet including data is actually edited into the transfer frame. Further, fluctuations in time from generation up to transfer of data to the ground station becomes larger, so that it is difficult to decide the time at which the data was generated from the time at which the data is received.
In a system in which time is required to be acquired by the a device which manages the time in the satellite, non-uniformity of around a couple of tens of microseconds is included in association with realization of a protocol for acquiring time, so that an error which is not desirable in a system requiring accurate time may be generated.
High precision time determination can be realized with GPS shown in FIG. 15 having been employed and becoming popular in recent years. However, the system configuration is very complicated. Further, in the example shown in FIG. 15, because the satellite 105 itself rotates around the Earth at a high speed a Doppler shift is generated. This Doppler shift makes the use of GPS on the ground difficult.
To solve the problems as described above in the conventional technology, it is an object of the present invention to provide a time synchronization system capable of determining time of generation of data with high precision and also with a simple configuration. It is also an object of the present invention to provide, a satellite system applied in the time synchronization system, a ground system applied in the time synchronization system, a time synchronization method, and a computer-readable recording medium with program for making a computer execute the method recorded therein.
In the present invention, the satellite system inserts a time frame to be used for establishing a time correlation between the satellite system and the ground system between the transmission frames at an arbitrary timing, and the ground system acquires the time of generation of data in the satellite system from this time frame. Thus, a time correlation between the time in the satellite and that on the ground can be established only by using the time frame, so that time of generation of data can precisely be determined with a simple configuration.
Further, the transmission frame is generated by packetizing the data generated in the satellite system, so that even complicated data processing can be executed.
Further, the satellite system generates the transmission frame utilizing the observation data generated in a plurality of equipments mounted thereon, so that the ground system can acquire a result of observation in the satellite.
Further, the satellite system distributes a time clock to each of the equipment and synchronizes the internal time in satellite (satellite time) with of the internal time in each of the equipments, so that a centralized time management in the satellite system can be realized.
Further, the satellite system appends the satellite time to the data generated in each of the equipment, so that time management of data can be realized in the satellite system.
Further, the satellite system generates the satellite time by counting clocks which are synchronized to a bit rate of the transmission frames, and set an entire portion of the satellite time below the time required for transmission of one frame to zero at the head of a transmission frame, so that a fraction of satellite time is eliminated and precision in time synchronization can be improved.
Further, in the satellite system, satellite time is corrected depending upon the changes in the temperature, so that time error due to temperature conditions inside the satellite system can be suppressed.
Further, the satellite system inserts satellite time at two different timings between the transmission frames, while the ground system acquires an average frequency of satellite time from the time interval between these two satellite times inserted between the transmission frames and a time interval between the ground times corresponding to these two satellite times. Then, the ground system corrects the time of generation of data in the satellite from the above average frequency and the amount of drift from the official frequency of the satellite time. With this, error in time progress in the satellite system and that in the ground system can be corrected, which makes it possible to acquire accurate time of generation of data in the satellite system.
Further, the satellite system estimates the amount of change in the frequency to be used for acquiring the satellite time according to a change of the internal temperature, and the ground system corrects the time of generation of data in the satellite system taking into account the estimated change in the frequency. Thus, the ground system can acquire the accurate time of generation of the data by taking into account the satellite time in the satellite system.
In the present invention, a time frame to be used for establishing a time correlation between the satellite system and the ground system is generated, and this time frame is inserted, when the frames are transmitted, between the transmission frames at an arbitrary timing to generated a transfer frame. Therefore, time correlation between the satellite time and the ground time can be established in the ground system utilizing this time frame.
Further, a convolution processor is provided for generating the transfer frames by packetizing the generated data, so that even complicated data processing can be executed.
In the present invention, the transmission frame is generated utilizing the observation data generated in a plurality of equipments mounted on the satellite system, so that a result of observation by the satellite system can be provided to the ground system.
Further, satellite time preserved in the satellite system is synchronized with the internal time in each of the equipment by distributing time clocks to each equipment, so that monolithic time management can be realized in the satellite system.
Further, satellite time is appended to data generated in each of the equipment, so that time management for each data can be realized in the satellite system.
Further, satellite time is generated by counting clocks each synchronized to a bit rate of the transfer frames, and an entire portion of satellite time below the time required for transfer of one frame is set to zero at the head of the transfer frames, so that a fraction of satellite time is eliminated and precision in synchronization can be improved.
Further, satellite time is corrected according to the changes in the temperature, so that a time error due to temperature conditions inside the satellite system can be suppressed.
In the present invention, time of generation of data in the satellite system is acquired according to a time frame inserted between received frames to be used for establishing a time correlation between the satellite system and the ground system. Thus, time correlation between the satellite system and ground system can be realized according to the time frame obtained from the satellite system, so that time of generation of data can precisely be determined with a simple configuration.
Further, an average frequency of satellite time is acquired from the time interval between two different satellite times and the time interval between the ground times corresponding to the two satellite times, and time of generation of data in the satellite system is corrected from this average frequency as well as from the amount of drift from then official frequency of satellite time. With this, error between time progress in the satellite system and that in the ground system can be corrected, so that an accurate time of generation of data in the satellite system can be acquired.
Further, time of generation of data in the satellite system is corrected by taking into account the change in the frequency estimated in the satellite system, so that accurate time of generation of data can be acquired by taking into account the satellite time in the satellite system.
In the present invention, a time frame to be used for establishing a time correlation between the satellite system and the ground system is generated, and a transfer frame is generated, when frames are transmitted, by inserting the generated time frame at an arbitrary timing between the transmission frame. Thus, time correlation between the satellite time and ground time can be established based on this time frame, so that accurate time of generation of data can precisely be determined with a simple configuration.
In the present invention, when receiving the frames, time of generation of data in the satellite system is acquired from the time frame inserted between the received frames to be used for establishing a time correlation between the satellite system and the ground system, and data is analyzed according to the time of generation of data. Then a time correlation between the satellite system and ground system can be established according to the time frame obtained from the satellite system and data can be analyzed using an accurate time, so that a result of data analysis can be acquired based on accurate time in the satellite system.
In the present invention, processing for generation of a time frame to be used for establishing a time correlation between the satellite time and the ground system is executed, and then a transfer frame is generated by inserting, when transmitting the frames, this time frame at an arbitrary timing between the transmission frames. Thus, time correlation between the satellite system and the ground system can be established according to this time frame by using a computer program, so that time of generation of data can precisely be determined with a simple configuration.
In the present invention, when receiving the frames, time of generation of data in the satellite system is computed according to the time frame inserted between the received frames, to be used for establishing a time correlation between the satellite system and the ground system, and data is analyzed according to the computed time of generation of data. Then the data can be analyzed according to an accurate time by establishing a time correlation between the satellite time and the ground time according to the time frame obtained from the satellite system using a computer program, so that a result of analysis based on an accurate time can be acquired.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.